Simplified contactless test of MCM thin film I/O nets using a plasma

ABSTRACT

A gas panel plasma plate is used to detect shorts and opens on a thin film surface of a multilayer ceramic module (MCM) through biasing a circuit of the module through bottom surface module (BSM) pins to produce a glow within the plasma plate. A grounded plane is placed above the module to be tested, and the gap between the module and the plane is filled with a gas. A plasma discharge is created by biasing the circuit. The current produced at the BSM pin by the plasma discharge is monitored. The monitored current of the circuit under test is compared to a current range of a known good module. In the alternative, the light flux produced by the plasma discharge is monitored, and the monitored light flux is compared to a light flux range of a known good module.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to testing electronic circuitsand, more particularly, to a contactless method and apparatus fortesting for opens and shorts in thin film networks connected to a bottomsurface metallurgy (BSM) input/output (I/O) pad on a multi-chip module(MCM) containing electrical thin film wiring nets or laser vias undertest.

2. Background Description

During the manufacture of the thin film layers of MCM substrates, it maynot be possible to utilize contact test methods until all top surfacemetallurgy (TSM) thin film layers of a substrate are completed. Foryield reasons, it may be desirable to locate shorts and opens on nets aseach thin film layer is made and repair them where possible.

General shorts and opens may be detected by optical inspectiontechniques with some success and some amount of escapes. MCM substratesmay tolerate some level of shorts and opens as many substrates are madewith the possibility of adding or deleting wiring after the part iscompleted or almost completed. Automatic optical inspection (AOI) may becostly or may not always be effective and sometimes test methods aremore reliable.

Not all shorts or opens are equally serious due to many MCM substratesbeing designed for repairability. In general, if a short or open occurson a net connected to a BSM I/O pad, it would be more serious and likelya fatal defect as it would not be possible to wire around or delete it.It is therefore desirable for yield reasons to find and locate shortsand opens on thin film nets connected to BSM I/O pads with special careand as effectively as possible.

Capacitive methods have had some success for some products where acapacitive plate is placed over the part and the capacitance at each BSMI/O pad is measured and compared to a learned value. A higher or lowercapacitance reading may indicate a short or open respectively using sucha test. One drawback to such a test is that the capacitance will beinfluenced by the brick line together with the thin film pattern undertest. If the brick line is very long, the percentage of the capacitancedue to the thin film portion of the net may be small and it may not bepractical or possible to efficiently find opens in the thin film netunder test, so the capacitance approach may be effective for shorts butfor very complex products may not always be as effective for opens.

Another class of fatal opens would be those which occur in the laservias between metalization layers and, as before, if a laser via on a BSMI/O net was too small or clogged, it may be a fatal defect. A capacitiveapproach would be no help in testing laser vias on BSM I/O nets. Opticalapproaches to laser via inspection may be problematic due to theexceedingly small size of laser vias, the variability of the appearanceof the surface of the metal at the bottom of the via and the need tocatch small amounts of residual polyimide at the bottom of the via. Inthe case of laser via inspection, it may be less possible to repair abad laser via, but it would be valuable for process diagnostics andyield learning to locate defective laser vias.

In some cases, it may not be possible to directly contact the BSM padsof an MCM substrate since for some products, the BSM is sealed inpolyimide to prevent or minimize a variety of manufacturing problems andcapacitive tests which require direct contact with a BSM pad may not bepossible.

Some have applied plasma techniques to inspection which also offer anoncontact test to the most general classes of TSM shorts and opens.These approaches tend to drive mechanical, optical complexity and mayrequire image processing.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide asimplified contactless method and apparatus for testing for opens andshorts of a device under test containing thin film electrical circuitwiring patterns connected to a BSM I/O pad.

It is another object of the invention to test laser vias connected to aBSM I/O pad.

According to the invention, a gas panel plasma plate is used to detectshorts and opens on a thin film surface of an MCM through biasing acircuit through bottom surface metallurgy (BSM) pins to produce a glowwithin the plasma plate. A grounded plane is placed above the module tobe tested, and the gap between the module and the plane is filled with agas. A plasma discharge is created in the gas by biasing a circuit inthe module. The current produced at the BSM pin by the plasma dischargeis monitored. The monitored current of the circuit under test iscompared to a current range of a known good module. In the alternative,the light flux produced by the plasma discharge is monitored, and themonitored light flux is compared to a light flux range of a known goodmodule.

Some MCM processes require that the BSM pads be sealed in polyimide formany of the processing steps, making it impossible to directly ohmicallycontact each I/O or voltage plane pad, although it may be possible tomake a small number of voltage plane pads specially dedicated to beingohmically probed by a pogo pin with special openings in the BSMpolyimide around them for that purpose. With this type of process, allBSM I/O pads are sealed in polyimide and not accessible by resistiveohmic pogo pin type probing. In this case, capacitive probing can beused through the polyimide. An interposer is placed parallel and beneaththe BSM. The interposer is insulating except that it includes aconductive cylinder under each BSM I/O pad. Each conductive cylinder hasa radius matching or slightly less than the BSM I/O pads. Eachconductive cylinder, with its matching BSM I/O pad, forms a parallelplate capacitor and, in the 1 to 20 MHZ range, can be used tocapacitively test for the presence of shorts. In this case, rather thanprobing the MCM BSM directly, the interposer which is pressed close andparallel to the MCM BSM is probed.

These tests can be mechanically simplified to detect a voltage plane toBSM signal I/O short which is a very significant cause of poor yieldsfor power mesh layers. With one optical or current measurement, thesefixtures detect if a power to BSM I/O short is present on a part beingfabricated. When ohmic contact with BSM I/O pads can be made with pogopins, it is also possible to detect the location of voltage plane to BSMI/O shorts magnetically. The difficulty again is presented if the BSMI/Os are sealed in polyimide through most of the process. In that case,a capacitively coupled interposer is again used to make contact with theBSM I/O pads at a high frequency. In this case, the interposer is asolid conductive sheet with holes in registration with voltage andground pads. These holes have a radius the same or slightly larger thanthe BSM voltage and ground pads. The conductive sheet with holesmaximizes capacitive coupling to the BSM I/O pads while minimizingcoupling to the voltage plane pads. As before, a few special pads areset aside for direct ohmic resistive contact probing of each voltageplane. These special pads are not sealed. In the case of testing powermesh layers for BSM I/O voltage shorts, the plasma induced current willbe much lower if no I/O is shorted to the very large area voltage plane,so one measurement of the one induced current will detect the presenceof any BSM I/O to voltage plane short of sufficiently low resistance.This test needs to occur at a high enough frequency to capacitivelycouple the plate to the BSM pads and needs to be in the 1 to 20 MHZrange.

Lastly, once a power to power or power to BSM I/O short is detected, alow frequency pulsed current is added to cause a blinking effect in theplasma to isolate where the short is on the TSM. The location of theshort needs to be identified in order to try to save the part by laserrepair. Since the current through the short will generally be in thehorizontal plane and the moving charges are in vertical motion, themagnetic field will exert a force locally on the glow plasma causing ablinking effect which can be inspected to locate the short for eventualrepair.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a functional block diagram showing the basic structure of thetesting apparatus according to the preferred embodiment of theinvention;

FIG. 2 is a functional block diagram showing the structure of analternative embodiment of the invention;

FIG. 3 is a side view in cross-section of one form of mechanical fixturethat may be used with either of the embodiments of FIGS. 1 and 2;

FIG. 4 is a side view in cross-section of another form of mechanicalfixture that may be used with either of the embodiments of FIGS. 1 and2;

FIG. 5 is a side view in cross-section of a test fixture showing a highfrequency signal capacitively coupled to BSM signal lines to test foropens and shorts where the BSM is sealed in polyimide;

FIG. 6 is a pictorial view of an ohmic test fixture for BSM I/O tovoltage plane shorts or voltage plane to voltage plane shorts;

FIG. 7 is a pictorial view of a plasma in line test fixture for voltageplane to BSM I/O shorts;

FIG. 8 is a pictorial view of a test fixture illustrating a method ofvisualizing a location of a voltage plane to voltage plane short usingan AC glow plasma with a pulsed current through the short;

FIG. 9 is a pictorial view of a test fixture illustrating a method ofvisualizing a location of a BSM I/O to voltage plane short using an ACglow plasma while pumping a pulsed current through the short; and

FIG. 10 is a pictorial view of a test fixture illustrating a method ofvisualizing a location of a BSM I/O to voltage plane short using an ACglow plasma while pumping a modulated plasma current through the short.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The invention is described in terms of three groups of methods that leadfrom one to the next in an ordered progression. The first group isillustrated by FIGS. 1 to 5 provides general in line test for opens andshorts for metalization layers and laser via layers. The second group,illustrated by FIGS. 6 and 7, provides for simplified shorts detectionfor power mesh layers. Finally, the third group provides for shortslocation for parts known to have a short in a power mesh layer and isillustrated in FIGS. 8 to 10.

Referring now to the drawings, and more particularly to FIG. 1, there isshown a substrate 11, such as a multi-layer ceramic module (MCM), overwhich a quartz plate 12 has been placed. The quartz plate 12 is spacedapart from the substrate 11 by a spacer 13. The spacer 13 provides aseal between the quartz plate 12 and the substrate 11. The space betweenthe substrate 11 and the quartz plate 12 is filled with a gas, such asneon or argon. The surface of the quartz plate 12 closest to thesubstrate has thin film of indium tin oxide deposited thereon. This filmis optically transparent but electrically conductive. The indium tinoxide film is connected to electrical ground. A direct current (DC) bias14 is applied to each BSM I/O pin 15 (only one shown) in a sequentialorder. The BSM I/O pin 15 is connected through the MCM substrate 11,through vias and layer metalization, to a corresponding TSM I/O net 16.The DC bias applied BSM I/O pin 15 causes a plasma discharge 17 in thegas between the substrate and the quartz plate 12. A current monitor 18monitors the current flow due to the plasma discharge. In thealternative, a low frequency voltage can be used in place of the DCbias. The use of a low frequency bias will accomplish the same thing asthe DC bias but additionally allow the use of a lock in amplifier ifdesired to improve the signal to noise ratio of the output measurement.

In FIG. 2, the current monitor is replaced by an inexpensive lens 21 andphotomultiplier tube 22. The photomultiplier tube 22 monitors the lightflux produced as a result of the plasma discharge, this light flux beingthe optical analog of the current sensed in the embodiment shown in FIG.1.

FIG. 3 illustrates a mechanical fixture which can be used with either ofthe embodiments of FIGS. 1 or 2. The fixture comprises a plate 31 havingan aperture formed therein to receive the quartz plate 12. The quartzplate 12 is sealed within the aperture to form a gas tight seal. On thebottom side of the plate 31 there is a rabbet formed about the peripheryof the aperture to receive the substrate 11. The faces of the rabbet arecovered with a gasket material 32 to form a gas tight seal with thesubstrate 11 under mechanical pressure. Two ports 33 (only one showing)communicate through the plate 31 with the space between the quartz plate12 and the substrate 11. These ports are used to first of all evacuateair from the space between the quartz plate 12 and the substrate 11 andthen to inject a luminescing gas, such as neon or argon, into the spacewhich will be at a slight vacuum on the order of 0.1 to 0.5 atmospheres.After the test, the gas can then be evacuated, to be recycled for thenext test, and air injected to break the vacuum and allow the substrateto be removed from the fixture. Once the substrate is mounted in thefixture and the luminescing gas injected in the space between the quartzplate 12 and the substrate 11, a robotic contact probe 34 is moved andcontacts each BSM I/O pad in a predetermined sequence to apply the DCbias to the BSM I/O pads. Depending on which embodiment is beingpracticed (i.e., FIG. 1 or FIG. 2), a current monitor measures thecurrent due to the plasma discharge or a photomultiplier tube generatesa signal proportional to the light flux due to the plasma discharge.

An alternative fixture is shown in FIG. 4. This fixture is essentiallythe same as that of FIG. 3 except that the robotic contact probe 34 isreplaced by a "pogo" contact fixture 35 having one pin for each BSM I/Opad. The pins of this fixture are brought into contact with the BSM I/Opads and then, through switching matrix 36, a DC bias (or low frequencyAC bias) is applied to each of the BSM I/O pads, again according to apredetermined sequence.

FIG. 5 shows a high frequency implementation of the plasma test foropens and shorts using capacitive coupling to the BSM pads. Thisapproach is used for those MCM processes which require that the BSM padsbe sealed in polyimide 41 for many of the processing steps, thus makingit impossible to directly ohmically contact each I/O or voltage planepad. An interposer plate 42 is place parallel and beneath the BSM. Theinterposer plate 42 is itself insulating but contains a plurality ifconductive cylinders or posts 43 under each BSM I/O pad. Theseconductive posts 43 have a radius which is the same or slightly lessthan the radius of the corresponding I/O pad 15. The interposer plate 42is separated from the BSM by a spacer defining an air gap between thepolyimide covered BSM and the interposer plate. The conductive posts 43and BSM pads 15 form parallel plate capacitors. A test signal having afrequency of 1 to 20 MHz is used to capacitively test for the presenceof shorts. This arrangement will test metalization lines for opens andshorts, but it will not test laser vias.

The next group of tests, shown in FIGS. 6 and 7, is a simplifiedprocedure of the embodiment shown in FIG. 5. The approach taken avoidsrobotic probing and a big switching matrix, but it is only good forpower layers, which are still almost half the layers in a typical MCM.The tests made with the fixtures shown in FIGS. 6 and 7 will detect ashort but will not locate it.

Referring first to FIG. 6, the interposer plate 45 comprises a solidconducive sheet 46 with holes 47 in registry with BSM voltage or groundpads 15. Pogo pins or other ohmic probes 48 pass through the holes 47 tomake ohmic contact with the pads 15. The conductive sheet 46 maximizescapacitive coupling to the BSM I/O pads while minimizing coupling tovoltage plane pads.

As mentioned, this simplified test fixture will test for BSM I/O tovoltage plane shorts or voltage plane to voltage plane shorts with oneoptical or current measurement. The test fixture will not, by itself,allow for location of detected shorts. However, it is possible, using amagnetic probe proximate the TSM, to detect the magnetic location ofvoltage plane to BSM I/O shorts.

Again, there is a difficulty in making the test if the BSM pads aresealed in a polyimide. A capacitively coupled interposer plate 51 asshown in FIG. 7 may be used in this case. The interposer plate 51comprises a solid conducive sheet 52 with holes 53 in registry with BSMvoltage or ground pads 15. As in the fixture of FIG. 5, a few specialpads, which are not sealed, are set aside for direct ohmic resistivecontact probing of each voltage plane. In the case of testing power meshlayers for BSM I/O voltage shorts, the plasma induced current will bemuch lower if no I/O is shorted to the very large area voltage plane soone measurement of the one induced current will detect the presence ofany BSM I/O to voltage plane short of sufficiently low resistance. Thistest is performed with a test signal frequency in the range of 1 to 20MHZ range to capacitively couple the plate 52 to the BSM pads.

While the test fixtures shown in FIGS. 6 and 7 do not provide a locationof detected shorts, they can be further modified as shown in FIGS. 8 to10 to provide an indication of location of detected shorts. FIG. 8 issimilar to FIG. 6 where ohmic probing is possible and illustrates amethod of visualizing the location of a voltage plane to voltage planeshort using an AC glow plasma with a pulsed current through the short.The current through a short 55 once detected is pulsed at a lowfrequence (e.g., on the order of 5 Hz). Since the current through theshort in the TSM will generally be in the horizontal plane and themoving charges are in vertical motion, the magnetic field will exert aforce locally on the glow plasma 56 causing a blinking effect which canbe visually inspected to localize the short and repair. Visualinspection is made through the indium tin oxide film on the quartz plate12.

In the case where the BSM I/O pads are sealed with a polyimide film, thetest fixture of FIG. 9, corresponding to the test fixture shown in FIG.7, is used. Again a pulsed AC current is applied to the detected shortto produce a blinking plasma which may be located by visual inspection.

As a further modification of the method illustrated in FIG. 9, thepulsed current is modulated by a high frequency signal as shown in FIG.10 to provide capacitive coupling to the BSM I/O pads. Again, ohmiccontact is made to at least one of each voltage plane pad not sealed inpolyimide.

While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Having thus described my invention, what I claim as new and desire tosecure by letters patent is as follows:
 1. A contactless test apparatusfor testing for opens and shorts in a module under test containingelectrical circuits comprising:a fixture frame having an aperture with arabbeted edge to receive a module to be tested; a gasketed seal withinthe rabbeted edge to form a gas tight seal with the module to be tested;a quartz plate sealed within the aperture and spaced from a top surfaceof the module to be tested when inserted into the fixture, the quartzplate being plated with a thin conductive coating to form a groundplane; means for filling a gap between the module to be tested and thequartz plate with a gas; biasing means for biasing a circuit in themodule to be tested to cause a plasma discharge in the gas; andrecording means for recording an effect of an open or short in saidcircuit on a glow pattern of the plasma discharge.
 2. The contactlesstest apparatus as recited in claim 1 wherein the gas is selected fromthe group consisting of neon and argon.
 3. The contactless testapparatus as recited in claim 1 wherein the thin conductive coating onthe quartz plate is indium tin oxide.
 4. The contactless test apparatusas recited in claim 1 wherein the effect of the glow pattern is acurrent flow and the recording means comprises a current monitor formonitoring the current flow through the circuit.
 5. The contactless testapparatus as recited in claim 1 wherein the effect of the glow patternis a light flux and the recording means comprises a means for measuringthe light flux.
 6. The contactless test apparatus as recited in claim 1wherein the biasing means includes a robotic probe for contacting padson a bottom surface of metallurgy of the module in a predeterminedsequence.
 7. The contactless test apparatus as recited in claim 1wherein the biasing means comprises:a plurality of probes for contactingpads on a bottom surface of metallurgy of the module; and switchingmeans for connecting each probe of the plurality of probes to a sourceof bias current in a predetermined sequence.
 8. The contactless testapparatus as recited in claim 1 wherein the biasing means comprises:aninterposer plate having cylindrical conductive posts in registry withpads on a bottom surface metallurgy of the module; and a source of highfrequency test signal selectively coupled to said conductive posts tocapacitively couple to the pads in a predetermined sequence.
 9. Thecontactless test apparatus as recited in claim 1 wherein the biasingmeans makes ohmic contact with at least selected voltage plane pads on abottom surface metallurgy of the module.
 10. The contactless testapparatus as recited in claim 1 further comprising means for pulsing abias current to a detected short to cause the glow pattern of plasmadischarge to blink thereby allowing visual inspection of the glowpattern to determine a location of the short.